M & Ns... Not appealing anymore?

Let m = 200 7 2008 m=2007^{2008} . Determine the number of positive integers n n with n < m n<m such that n ( 2 n + 1 ) ( 5 n + 2 ) n(2n+1)(5n+2) is divisible by m m . (VMO, 2008)


The answer is 8.

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1 solution

Patrick Corn
May 13, 2014

This seems incorrect. The answer should be 8 8 . If you restricted to n m n \le m or nonnegative integers n n , then 9 9 would be correct.

200 7 2008 = 3 4016 22 3 2008 2007^{2008} = 3^{4016} \cdot 223^{2008} is the prime factorization. But it's clear that only one of the three factors can be divisible by 3 3 and only one can be divisible by 223 223 , so one of the factors n , 2 n + 1 , 5 n + 2 n, 2n+1, 5n+2 has to be divisible by 3 4016 3^{4016} and one of them has to be divisible by 22 3 2008 223^{2008} . This is necessary and sufficient for the product to be divisible by m m .

So this is a Chinese Remainder Theorem computation. There are 9 9 total solutions mod m m , since there are 3 3 choices for the factor in both conditions, but one of the solutions is 0 0 , which is ruled out by the problem. So the correct answer is 8 \fbox{8} .

Yeah exactly I put 9 and was very confused.

Finn Hulse - 7 years, 1 month ago

WELL,DIDN'T SEEM A LEVEL5 QUESTION TO ME

A Former Brilliant Member - 6 years, 8 months ago

Ohh sorry! Wasn't thinking when I put the solution there.

Joshua Ong - 7 years ago

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